Process for preparing (meth)acrylates

ABSTRACT

Process for preparing (meth)acrylates of the formula (I) 
       CH 2 ═C(R 1 )—CO—O—R 2    (I)
         in which R 1  is hydrogen or methyl and   R 2  is a saturated or unsaturated, linear or branched, aliphatic or cyclic alkyl radical having 6 to 22 carbon atoms, or a (C 6 -C 14 )-aryl-(C 1 -C 8 )-alkyl radical;   by reacting a (meth)acrylate of the formula II       

       CH 2 ═C(R 1 )—CO—OR 3    (II)
         with an alcohol of the formula (III)       

       HO—R 2    (III)
         in the presence of an amount of a suitable catalyst which catalyses the reaction and of an amount of a phenolic polymerization inhibitor or a combination of two or more phenolic polymerization inhibitors which is sufficient to inhibit undesired polymerization;   the reaction being undertaken with input or introduction into the reaction mixture resulting from the reaction of an amount of oxygen or of an oxygenous gas mixture sufficient to inhibit undesired polymerization, and the process is characterized in that   the specific total oxygen input is less than or equal to 1.0 1/kg, measured in litres of oxygen per kilogram of (meth)acrylate of the formula (I), where the volume of oxygen introduced is calculated at a temperature of 25° C. and a pressure of 101 325 pascal.       

     The resulting (meth)acrylates can surprisingly be processed to particularly high molecular weight emulsion polymers which are, for example, outstandingly suitable for use as flow resistance reducers in mineral oil extraction.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. application Ser. No.12/745,703, filed on Jun. 2, 2010, which is a 35 U.S.C. 371 nationalstage patent application of international patent applicationPCT/EP08/062982, filed Sep. 29, 2008, the text of which is incorporatedby reference, and claims priority to U.S. Provisional Application No.61/014,927, filed on Dec. 19, 2007, the entire content of which isincorporated herein by reference.

The present invention relates to a process for preparing(meth)acrylates, to (meth)acrylates prepared by this process and totheir use for preparing high molecular weight homopolymers orcopolymers.

Some processes for preparing (meth)acrylates are known (DE 3423441, DE3430446, U.S. Pat. No. 5,072,027). (Meth)acrylates can be used, interalia, as monomers to prepare polymer dispersions. What is especiallydesirable is the preparation of particularly high molecular weightpolymer dispersions, since they can be used, for example, as flowresistance reducers in mineral oil extraction.

It is therefore an object of the present invention to provide analternative process for preparing (meth)acrylate monomers which enablesthe monomers obtained to be particularly advantageously suitable forpreparing high molecular weight polymer dispersions.

The present invention provides a process for preparing (meth)acrylatesof the formula (I)

CH₂═C(R¹)—CO—O—R²   (I)

-   -   in which R¹ is hydrogen or methyl and    -   R² is a saturated or unsaturated, linear or branched, aliphatic        or cyclic alkyl radical having 6 to 22 carbon atoms, or a        (C₆-C₁₄)-aryl-(C₁-C₈)-alkyl radical;    -   by reacting    -   a (meth)acrylate of the formula II

CH₂═C(R¹)—CO—OR³   (II)

-   -   in which R¹ and R³ are each independently hydrogen or methyl;    -   with an alcohol of the formula (III)

HO—R²   (III)

-   -   in which R² is a saturated or unsaturated, linear, branched or        cyclic alkyl radical having 6 to 22 carbon atoms, or a        (C₆-C₁₄)-aryl-(C₁-C₈) -alkyl radical;    -   in the presence of an amount of a suitable catalyst which        catalyses the reaction; and    -   in the presence of an amount of a phenolic polymerization        inhibitor or a combination of two or more phenolic        polymerization inhibitors which is sufficient to inhibit        undesired polymerization;    -   the reaction being undertaken with input or introduction into        the reaction mixture resulting from the reaction of an amount of        oxygen or of an oxygenous gas mixture sufficient to inhibit        undesired polymerization,    -   characterized in that    -   the specific total oxygen input is less than or equal to 1.0        l/kg, measured in litres of oxygen per kilogram of        (meth)acrylate of the formula (I), where the volume of oxygen        introduced is calculated at a temperature of 25° C. and a        pressure of 101 325 pascal.

The R² radical in the alcohol of the formula (III) is understood, forexample, to mean a hexyl, heptyl, octyl, 2-octyl, 2-ethylhexyl, nonyl,2-methyloctyl, 2-tert-butylheptyl, 3-isopropylheptyl, decyl, undecyl,5-methylundecyl, dodecyl, stearyl and/or behenyl radical, and/or acycloalkyl radical such as cyclohexyl, tert-butylcyclohexyl,cycloheptyl, cyclo-octyl, bornyl and/or isobornyl.

R² is preferably a linear or branched alkyl radical having 8 to 12carbon atoms, particular preference being given to the 2-ethylhexylradical.

Moreover, the R² radical may be an optionally substituted(C₆-C₁₄)-aryl-(C₁-C₈)-alkyl radical, preferably a(C₆-C₁₂)-aryl-(C₁-C₄)-alkyl radical, for example the benzyl,naphthylmethyl, naphthylethyl, 2-phenylethyl, 2-phenoxyethyl,4-phenylbutyl, 3-phenylbutyl, 2-phenylbutyl and/or the 2-biphenylethylradical. Particular preference is given to the benzyl, 2-phenylethyland/or 2-phenoxyethyl radical.

Alcohols of the formula (III) are known and are commercially available,for example, from Dow, Shell, Clariant or EOXO.

It is also possible to use mixtures of alcohols which stem fromrenewable raw materials or are obtainable from industrial synthesisprocesses, especially preferably mixtures of alcohols having n- andisoalkyl radicals having 6-22 carbon atoms.

The (meth)acrylate of the formula (II) is preferably methyl(meth)acrylate or methacrylic acid, preferably methyl methacrylate.

(Meth)acrylates of the formula (II) are obtainable commercially, forexample from Röhm. Preference is given to the preparation of(meth)acrylates of the formula (I) in which R¹ is methyl.

The weight ratio of the alcohol of the formula (III) to the(meth)acrylate of the formula (II) is preferably in the range of 1:1.5to 1:10, more preferably 1:2.5 to 1:5 and most preferably in the rangeof 1:3 to 1:4. Too small an excess can reduce the reaction rate; toogreat an excess is economically unviable since it reduces the utilizabletank volume.

To catalyse the present esterification or transesterification, it ispossible to use catalysts, for example tetraisopropyl titanate,tetrakis(ethyl-hexyl) titanate, zirconium acetylacetonate, a dialkyltincompound, at least one lithium compound selected from the group oflithium oxide, lithium hydroxide, and lithium chloride, optionally incombination with a calcium compound selected from the group of calciumoxide and calcium hydroxide, or an acid (e.g. p-toluenesulphonic acid,sulphuric acid, methanesulphonic acid).

Preference is given to using tetraisopropyl titanate ortetrakis(ethylhexyl) titanate. These are commercially available, forexample from Du Pont or Johnson Matthey Catalysts. The CAS number ofzirconium acetylacetonate is 17501-44-9. The preparation of zirconiumacetyl-acetonate from acetylacetone (pentane-2,4-dione) and zirconiumcompounds is described, for example, in Houben-Weyl, Methoden derorganischen Chemie [Methods of organic chemistry], 4th edition, Vol.VI/2, 1963, pages 53-55 and 58 to 61, and in A. E. Martell, M. Calvin,“Die Chemie der Metallchelatverbindungen” [The chemistry of metalchelate compounds] (1958).

Advantageously, it is possible to use 0.2 to 10 mmol, more preferably0.5 to 8 mmol, of catalyst per mole of alcohol of the formula (III).

The catalyst can also be prepared in situ, in which case the startingmaterials can be added to the reaction mixture before or during theesterification or transesterification.

The reaction can be effected at elevated or reduced pressure. In aparticularly appropriate modification of the present invention, theesterification or transesterification can be performed at a pressure inthe range of 200 to 2000 mbar, more preferably in the range of 500 to1300 mbar.

The reaction temperature may, depending especially on the pressure,likewise be within a wide range. In a preferred embodiment of thepresent invention, the reaction is effected preferably at a temperaturein the range of 80° C. to 140° C., more preferably 85 to 125° C.

Particular advantages can be achieved when the temperature at which thereaction is effected is increased in the course of the reaction.

The esterification or transesterification can be performed batchwise,semi-batchwise or continuously, preference being given to the continuousreaction.

It is also possible to initially charge a portion of the (meth)acrylateused for the transesterification not at the start of the reaction butrather only during the reaction.

The process according to the invention can be performed in bulk, i.e.without use of a further solvent. If desired, it is also possible to usean inert solvent.

These include petroleum, benzene, toluene, n-hexane, cyclohexane andmethyl isobutyl ketone (MIBK), methyl ethyl ketone (MEK).

In the batchwise reaction regime of the process according to theinvention, it is particularly appropriate to mix all components, forexample the alcohol, the (meth)acrylate and the catalyst, and then toheat this reaction mixture to boiling.

In the case of transesterification, this first removes water which maybe present in the alcohol in azeotropic form with the ester of themethacrylic acid. In the case of certain catalysts, it may beadvantageous not to add the catalyst until after the water removal.

Subsequently, the alcohol released may be removed from the reactionmixture by distillation, if appropriate azeotropically. In theesterification, the water of reaction is removed, if appropriate as anazeotrope with a suitable azeotroping agent.

The pure reaction times are dependent upon factors including theparameters selected, for example pressure and temperature. They are,though, generally in the range of 1 to 10 hours, preferably of 1 to 5hours and most preferably 1 to 3 hours. In the continuous process, theresidence times are generally in the range of 0.5 to 5 hours, preferablyof 1 to 4 hours, even more preferably 1 to 3 hours, especially 1 to 2hours.

The reaction can preferably take place with stirring, in which case thestirring speed is more preferably in the range of 50 to 2000 rpm, mostpreferably in the range of 100 to 500 rpm.

In order to prevent undesired polymerization of the (meth)acrylates, aphenolic polymerization inhibitor or a combination of two or morephenolic polymerization inhibitors is used in the reaction. Thesecompounds, for example hydroquinones, hydroquinone ethers such ashydroquinone monomethyl ether or di-tert-butyl-pyrocatechol, orsterically hindered phenols, are widely known in the technical field andare generally commercially available.

Particular preference is given to using hydroquinone and/or hydroquinonemonomethyl ether.

The concentration of phenolic polymerization inhibitors in the(meth)acrylate of the formula I obtained by the process according to theinvention should be kept as low as possible in order firstly to reliablyprevent unwanted polymerization of the monomer and secondly to ensurethe desired preparation of polymers with ultrahigh molar masses. Thisrequirement is appropriately taken into account as early as in thepreparation process.

The concentration of phenolic polymerization inhibitors is preferably<50ppm, preferably<20 ppm, more preferably<15 ppm, especially<12 ppm(wt./wt.), based on the (meth)acrylate of the formula I.

The aforementioned concentration data are on the basis that the monomerhas not been removed beforehand by distillation nor have further processsteps been effected to remove the inhibitors (for example extraction,absorption on, for example, activated carbon or ion exchange resins).

When the monomer is removed by distillation or there follow processsteps for removing the inhibitor, the concentration of phenolicpolymerization inhibitors, according to the effectiveness of the processstep, may be even higher, for example 10-1000 ppm. In this case too, theamount is preferably kept as low as possible in order to reliablyprevent unwanted polymerization, i.e., for example, at 100-200 ppm(wt./wt.), lower if possible.

When the initiator is added, it is appropriate to charge not only thereaction vessel but also the column and, if appropriate, the condensersurfaces with inhibitors which can be metered, for example, into thecolumn reflux line.

Oxygen is additionally used for inhibition. It can be used, for example,in the form of air, in which case the amounts are advantageously meteredin such that the oxygen content in the gas phase is less than or equalto 18% oxygen (v/v), and is preferably below the explosion limit.

Particular preference is given to introducing an oxygenous gas mixtureof oxygenous lean air having a content of less than or equal to 5%oxygen (v/v) into the reaction mixture.

Inert gas-oxygen mixtures, for example nitrogen-oxygen, argon-oxygen orcarbon dioxide-oxygen mixtures, may likewise be used.

According to the invention, the specific total oxygen input is less thanor equal to 1.0 l/kg, measured in litres of oxygen per kilogram of(meth)acrylate of the formula (I). The oxygen volume introduced per unittime is calculated from the volume flow and the oxygen content of thegas mixture introduced at a temperature of 25° C. and a pressure of 101325 pascal. The volume flow of the gas mixture can be determined withsuitable measuring instruments, for example with variable area measuringinstruments (rotameters from Yokogawa). For the calculation of thespecific total oxygen input, the period during which oxygen isintroduced into the reaction mixture or the (meth)acrylate of theformula I at temperatures above 80° C. is employed.

The specific total oxygen input is preferably less than or equal to 0.5litre of oxygen per kilogram of product of the formula (I), even morepreferably less than or equal to 0.3 litre of oxygen per kilogram ofproduct of the formula (I), especially less than or equal to 0.2 litreof oxygen per kilogram of product of the formula (I).

In the case of plants with a customary order of magnitude via productionpoint of view (reactor volume ≧0.25 cbm-24 cbm), the oxygen isintroduced by introducing air, preferably via a tube (diameter at theexit point, for example, 0.5-2 cm) which reaches down to close to thebottom in the interior of the reaction tank. The gas introduced by meansof this apparatus flows through a liquid column of about 0.5-7 m whichconsists of the reaction mixture.

It has been found that, surprisingly, especially in the case of use ofthe above-described industrial or industrial scale reaction vessels, asignificant difference in the specific viscosities and in the molecularweights of the emulsion polymers prepared from the monomers is observedwhen different amounts of oxygen are introduced into the reactionmixture during the preparation of the monomers. The specific viscositiesand the molecular weights are increased to the desired degree when smallamounts of oxygen are introduced (less than or equal to 1.0 l/kg,measured in litres of oxygen per kilogram of (meth)acrylate). Whenlarger amounts of oxygen are added (greater than 1.0 l/kg, measured inlitres of oxygen per kilogram of (meth)acrylate), undesirably lowspecific viscosities or molecular weights are obtained.

In a particular preferred embodiment, the process according to theinvention is performed continuously and an oxygenous gas mixture ofoxygenous lean air having a content of less than or equal to 5% oxygen(v/v) is introduced into the reaction mixture.

According to an appropriate embodiment of the present invention, in thecase of the transesterification, the methanol released from the(meth)acrylate used can be removed by distillation. In this case, it isadvantageously possible, for example, to remove a mixture whichcomprises methyl (meth)acrylate and methanol. A portion of the mixtureremoved can advantageously be recycled into the next batch. In thismodification, the recyclable fraction of the mixture removed can beobtained towards the end of the reaction, especially after a conversionof 80%, preferably after a conversion of 90%. For example, theproportion of the mixture recycled to the start of the next batch may bein the range of 40 to 60%, based on the total weight of (meth)acrylateused.

In batchwise processes, excess reactant, especially the unconverted(meth)acrylate, can be removed by distillation towards the end of thereaction and be used again in the next batch without furtherpurification.

The methanol-rich distillate obtained at the start of atransesterification can likewise be recycled, for example byincorporation into a plant, operated in an integrated system, forpreparing the (meth)acrylate to be transesterified.

A suitable plant for performing the present esterification ortransesterification may, for example, be a stirred tank reactor with astirrer, steam heater, distillation column and condenser. Such plantsare known per se and are described, for example, in Ullmann'sEncyclopaedia of Industrial Chemistry (6th edition), publisher:Wiley-VCH, Weinheim 2003, volume 10, page 647. The size of the plantdepends on the amount of methacrylate to be prepared, and the processaccording to the invention can be performed either on the laboratoryscale (reactor volume 0.5-20 litres) or, particularly advantageously, onthe industrial scale. In a particular aspect, the stirred tank reactormay accordingly have a tank volume in the range of 0.25 m³ to 50 m³,preferably 1 m³ to 50 m³, more preferably 3 m³ to 50 m³. In the case ofthe particularly preferred continuous preparation, the tank volume ispreferably smaller and is, for example, 1-6 m³. The stirrer of thereactor tank can be configured especially in the form of an anchorstirrer, impeller, paddle stirrer or inter-MIG stirrer.

The task of the distillation column is to ensure that a methanol-richazeotrope is removed in order to minimize the losses of reactant esterwhich is inevitably also discharged. In the esterification, reactant andproduct components are retained for the benefit of the azeotropingagent-water azeotrope.

The distillation column may have one, two or more separating stages. Thenumber of separating stages refers to the number of trays in a traycolumn or the number of theoretical plates in the case of a column withstructured packing or a column with random packing. Examples of amultistage distillation column with trays include those such asbubble-cap trays, sieve trays, tunnel-cap trays, valve trays, slottrays, slotted sieve trays, bubble-cap sieve trays, jet trays,centrifugal trays; examples of a multistage distillation column withrandom packings are those such as Raschig rings, Lessing rings, Pallrings, Berl saddles, Intalox saddles; and examples of a multistagedistillation column with structured packings are those such as theMellapak type (Sulzer), the Rombopak type (Kühni), the Montz-Pak type(Montz). By virtue of the conversion-dependent adjustment of the refluxratio, it is possible, for example, in the case of use of methylmethacrylate, to establish a methanol content in the distillate which isabove 60% over wide ranges of conversion in the transesterification.

The suitable condensers which may be present in the plant for performingthe present esterification or transesterification include plate and tubebundle heat exchangers.

After the reaction has ended, the resulting (meth)acrylate in many casesalready satisfies the high requirements detailed above, such thatfurther purification is in many cases not necessary. However, theproduct will preferably be isolated by distillation after the reactionhas ended.

To further enhance the quality and especially to remove the catalyst,the resulting mixture can be purified by known processes. Owing to thepolymerization tendency of the monomer, it is advisable to employdistillation processes in which the thermal stress on the substance tobe distilled is minimized. Very suitable apparatus is that in which themonomer is evaporated continuously from a thin layer, such asfalling-film evaporators and evaporators with a rotating wiper system.Short-path evaporators can also be used. Such apparatus is known(Ullmann's Encyclopaedia of Industrial Chemistry (6th edition),publisher: Wiley-VCH, Weinheim 2003, volume 36, page 505). For example,a distillation can be performed, in which a continuous evaporator with arotating wiper system and attached column can be used. This distillationcan be performed, for example, at a pressure in the range of 40 to 60mbar and an evaporator temperature of 110° C. to 130° C.

The invention further provides a (meth)acrylate of the formula (I)obtained by the process claimed. It is characterized in that it containspreferably not more than 5 ppm, especially not more than 3 ppm, mostpreferably 1 ppm of polymerization inhibitors which do not require thepresence of oxygen to inhibit the polymerization.

Polymerization inhibitors which do not require the presence of oxygen toinhibit the polymerization are understood, for example, to meancompounds of the formula IV

where the R⁹ radicals are each independently a linear or branched alkylradical, preferably having 1 to 6, especially having 1 to 4 carbonatoms, such as a methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutylor a tert-butyl radical, especially a methyl radical. A compound of theformula (IV) is sold under the name4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl from Degussa GmbH andunder the brand Tempol® by Ciba. Further compounds are, for example,2,2-diphenyl-1-picrylhydrazyl, phenothiazine,N,N′-diphenyl-p-phenylenediamine, nigrosine (phenazine dye mixture),para-benzoquinone, or cupferron (ammonium salt ofN-nitroso-N-phenylhydroxylamine or of phenylnitroso-hydroxylamine).

In addition, the (meth)acrylate of the formula (I) obtained by theprocess claimed preferably contains not more than 20 ppm, preferably notmore than 15 ppm, most preferably 10 ppm of polymerization inhibitors.

The (meth)acrylates thus obtained can surprisingly be processed to giveparticularly high molecular weight homo- or copolymers which areoutstandingly suitable for thickening liquids or for adjusting theirflow properties, for example when used as flow resistance reducers inmineral oil extraction.

The invention therefore further provides for the use of at least one(meth)acrylate obtainable by the process according to the invention forpreparing homo- or copolymers having specific viscosities η_(spec/c)greater than or equal to 1000 ml/g, measured in THF. Particularpreference is given to the preparation of homo- or copolymers havingspecific viscosities η_(spec/c) greater than or equal to 1150 ml/g,especially η_(spec/c) greater than or equal to 1300 ml/g, measured inTHF. The monomer used is preferably 2-ethylhexyl methacrylate. Veryparticular preference is given to the preparation of 2-ethylhexylmethacrylate homopolymers. The specific viscosities η_(spec/c) aredetermined on the basis of DIN 51562 in THF as a solvent. Theconcentration should be selected such that a relative viscosity in therange of 1.1-1.2 is achieved.

The monomers prepared by the process according to the invention may becopolymerized with one another in any ratios, provided that theresulting copolymer has the claimed specific viscosity in THF. Inprinciple, it is also possible, though not preferred, to use a certainproportion of nonpolar non-acrylate monomers as comonomers to thegenerally nonpolar (meth)acrylate monomers, for example up to 50%,provided that they copolymerize sufficiently well with the(meth)acrylate monomers under the polymerization conditions used.Examples thereof are styrene, α-methylstyrene or long-chain vinyl esterssuch as vinyl versatate.

In this connection, “nonpolar” means the solubility of the monomers indemineralized water of <0.1 g/100 g at 20° C., without wishing to fixsuitability strictly to this value.

The person skilled in the art can find information regardingcopolymerization behaviour in standard works such as the PolymerHandbook ((4th edition), 1999, John Wiley & Sons).

What is crucial—and this is the case for polarity and copolymerizationbehaviour—is that the resulting copolymers have specific viscositiesη_(spec/c) of >1150 ml/g in THF. In this context, the use of morestrongly polar (meth)acrylates as a comonomer is also not preferred,though not completely ruled out.

Suitable comonomers, which are not, however, limited thereto, arespecified, for example, in WO 2006/073780 (pages 7-9), to whichreference is made explicitly.

Polyunsaturated monomers are unsuitable as comonomers since theycounteract expansion of the polymer aggregates by means of crosslinking.

Any customary polymerization process is suitable for preparing the homo-or copolymers, preference being given to emulsion polymerization.Processes for preparing high molecular weight (meth)acrylate polymers byemulsion polymerization are described, for example, in EP-A-555054,EP-A-882739 and WO 2006/081010.

The exact procedure in the emulsion polymerization of the inventive(meth)acrylate monomers of the formula I is uncritical, provided thatconditions which lead to stable dispersions and to polymers with highmolecular weights (η_(spec/c) values) and good solubility in THF aremaintained. The polymerization is preferably effected batchwise and isperformed at low temperatures especially in the presence of a redoxinitiator system.

A very low free-radical flow leads to the desired high η_(spec/c)values. On the other hand, undesirably long inhibition periods can ariseas a result in the case of inadequate oxygen exclusion and/or presenceof inhibitors which are effective without oxygen.

The reaction mixture is selected such that the fully polymerizeddispersion has a dry content of 20-65% by weight. The reaction mixtureto be polymerized contains generally 35-80, preferably 50-60 parts byweight of water, and a total of 20-65, preferably 40-50 parts by weightof monomer and emulsifier, where the proportions by weight specifiedplus that of the initiator system and that of any buffer present add upto 100.00 parts by weight.

Preference is given to using purified water such as distilled ordeionized water.

The reaction mixture may preferably also comprise at least one buffer.It is possible to use any buffer which is compatible with the initiatorsystem used, for example carbonate, phosphate and/or borate buffer, inthe generally customary amounts which are required to establish aparticular pH.

The mixture is stabilized by means of emulsifiers and optionally byprotective colloids.

The total amount of emulsifier is generally 0.1-10% by weight, 0.5-5% byweight, especially 0.5-3% by weight, based on the total weight of themonomer.

Particularly suitable emulsifiers are anionic or nonionic emulsifiers ormixtures thereof, especially:

-   -   alkyl sulphates, preferably those having 8 to 18 carbon atoms in        the alkyl radical, alkyl and alkylaryl ether sulphates having 8        to 18 carbon atoms in the alkyl radical and 1 to 50 ethylene        oxide units;    -   sulphonates, preferably alkylsulphonates having 8 to 18 carbon        atoms in the alkyl radical, alkylarylsulphonates having 8 to 18        carbon atoms in the alkyl radical, esters and monoesters of        sulphosuccinic acid with monohydric alcohols or alkylphenols        having 4 to 15 carbon atoms in the alkyl radical; these alcohols        or alkylphenols may optionally also be ethoxylated with 1 to 40        ethylene oxide units;    -   phosphoric partial esters and their alkali metal and ammonium        salts, preferably alkyl and alkylaryl phosphates having 8 to 20        carbon atoms in the alkyl or alkylaryl radical and 1 to 5        ethylene oxide units;    -   alkyl polyglycol ethers, preferably having 8 to 20 carbon atoms        in the alkyl radical and 8 to 40 ethylene oxide units;    -   alkylaryl polyglycol ethers, preferably having 8 to 20 carbon        atoms in the alkyl or alkylaryl radicals and 8 to 40 ethylene        oxide units, especially C₈-C₌alkylphenol ethoxylate;    -   ethylene oxide/propylene oxide copolymers, preferably block        copolymers, favourably having 8 to 40 ethylene oxide or        propylene oxide units.

To stabilize the polymer dispersion, preference is given to usingmixtures of anionic emulsifier and nonionic emulsifier, in which casethe anionic emulsifier is advantageously initially charged and thenonionic emulsifier is added, if appropriate not until after thepolymerization has ended. Mixtures of alkyl sulphates and C₈-C₁₂alkylphenol ethoxylate in a weight ratio of 0.7 to 1.3 have been foundto be very particularly useful.

Optionally, the emulsifiers may also be used in a mixture withprotective colloids. Suitable protective colloids include partlyhydrolysed polyvinyl acetates, polyvinylpyrrolidones,carboxymethylcellulose, methyl-cellulose, hydroxyethylcellulose,hydroxypropyl-cellulose, starches, proteins, poly(meth)acrylic acid,poly(meth)acrylamide, polyvinylsulphonic acids, melamine-formaldehydesulphonates, naphthalene-formaldehyde sulphonates, styrene-maleic acidand vinyl ether-maleic acid copolymers. If protective colloids are used,they are preferably used in an amount of 3 to 5% by weight, based on thetotal amount of the monomer.

The protective colloids may be initially charged before the start of thepolymerization or be metered in.

However, it should be ensured that the use of protective colloids doesnot impair the solubility in THF and the resulting specific viscosities.The use of protective colloids is therefore generally not preferred.

The initiation is effected with the initiators customary for emulsionpolymerization. Suitable organic initiators are, for example,hydroperoxides such as tert-butyl hydroperoxide or cumene hydroperoxide.Suitable inorganic initiators are hydrogen peroxide and the alkali metaland ammonium salts of peroxodisulphuric acid, especially sodiumperoxodisulphate, potassium peroxodisulphate or ammoniumperoxodisulphate. The initiators mentioned may be used either alone orin combination with one or more reducing components.

Preferred redox partners of the initiators are transition metal saltshaving two oxidation states, for example iron sulphate and/or ironammonium sulphate. When additional reducing components are used, forexample bisulphites, metabisulphites, ascorbic acid, isoascorbic acidand sodium formaldehydesulphoxylate, catalytic traces of the transitionmetals are generally sufficient to trigger the polymerization, forexample 10 ppm by weight based on the monomer. Without these components,generally higher concentrations of transition metals are required, forexample 100 ppm by weight.

The initiator can be initially charged or metered in. In addition, it isalso possible to initially charge a portion of the initiator and/or onecomponent of the initiator system and to meter in the remainder or theother component. Preference is given to the latter.

The molar ratio of monomer to initiator is generally selected at a levelas high as possible when the intention is to achieve high specificviscosities of the polymer. On the other hand, a required minimum amountof initiator arises from the requirements for a reliable and uniformpolymerization and the length of the inhibition period. This minimumamount depends on the monomer quality, the content of polymerizationinhibitors and on process conditions, such as the completeness of oxygenexclusion. It can be determined easily by a person skilled in the art bymeans of experiments.

The molar ratio of monomer to initiator is preferably 1×10³:1-5×10⁶:1,especially 1×10⁴:1-2×10⁶:1.

The molar ratio of the monomer to the reducing component is likewisepreferably 1×10³:1-5×10⁶:1, especially 1×10⁴:1-2×10⁶:1.

The polymerization is preferably effected by the batch process. Thepolymerization temperature is generally 0 to 40° C., preferably 0 to 20°C., especially 0 to 10° C. The polymerization should take place withexclusion of oxygen, preferably in an inert gas atmosphere. For thispurpose, an inert gas such as nitrogen is introduced continuously intothe vessel containing the reaction mixture. Good mixing of the reactionmixture with the aid of a suitable stirrer should be ensured.

In the preferred embodiment, the initial charge in the reaction vesselcomprises water, a buffer system, an anionic emulsifier, a firstinitiator component and the (meth)acrylate monomer, especially EHMA.

The polymerization is preferably started by adjusting the mixture to thepolymerization temperature and metering in a second initiator component,preferably dissolved in water. The addition time of the dissolvedinitiator is generally 5 to 20 h. After the end of addition, it ispossible to continue stabilization with a nonionic emulsifier.

In general, the monomer is polymerized up to a conversion of at least95.0% by weight, especially at least 99% by weight, based in each caseon the total weight of the monomer.

On completion of the polymerization, the (meth)acrylate homo- orcopolymer can be removed from the aqueous dispersion by generallycustomary physical methods (for example filtering, centrifugation). Ingeneral, the removal of the polymers is preceded by a coagulation step,for example by electrolyte addition.

The high molecular weight (meth)acrylates thus prepared can be used inthe form of aqueous dispersion directly as a thickener or as a dragreducer of crude oils and/or mineral oil fractions.

The examples which follow are intended to illustrate the inventionwithout restricting it thereto.

EXAMPLES Example 1 Preparation of 2-ethylhexyl methacrylate by a batchprocess

A 12 m³ stirred tank reactor with stirrer, steam heater, distillationcolumn and condenser is initially charged with 4200 kg of2-ethylhexanol, 5000 kg of methyl methacrylate (MMA), 0.840 kg ofhydroquinone monomethyl ether as an inhibitor and 28 kg oftetra-isopropyl titanate as a catalyst, which are stirred whileconstantly introducing air (14 m³/h).

To stabilize the column, over the entire reaction phase, a total of 160kg of MMA which contains 0.2 kg of hydroquinone monomethyl ether aremetered into the column reflux. The mixture is heated to boilingtemperature (beginning at approx. 90° C.), in the course of which thecolumn is initially operated with full reflux. As soon as thetemperature at the top of the column falls below 70° C., themethanol-MMA mixture formed is drawn off with variable reflux ratio(2:1-10:1). After approx. 3 hours and the removal of approx. 1200 l ofmethanol-MMA mixture, the reaction is very substantially complete(conversion >90%). As a result of the removal of the low-boilingcomponents, the product temperature has risen to 116° C.

Up to a product temperature of 130° C., excess MMA is subsequently drawnoff with a reflux ratio of 1:2 under standard pressure over a period ofabout 2 hours.

Thereafter, the MMA which still remains is removed completely underadjusted vacuum (1000-30 mbar) at a constant bottom temperature of 120°C. and without reflux. The air introduction is reduced to 4 m³/h in thevacuum phase. When no further MMA distillate is obtained with the bestvacuum over a period of 30 minutes, the vacuum is broken (duration about2 hours).

The vessel contents, consisting of the catalyst-containing 2-ethylhexylMA, are subsequently stabilized with 2.5 kg of Irganox 1076, and2-ethylhexyl MA is distilled off with a reflux ratio of 1:10 under thebest possible vacuum (approx. 30 mbar) and an average bottom temperatureof 130° C.-140° C. The air introduction of 4 m³/h is maintained; thedistillation step is complete after about 2 hours.

With a bottom residue of approx. 800 kg, 4900 kg of pure ester areobtained with the following composition (determined by gaschromatography):

2-ethylhexyl MA: 99.4%

-   -   2-ethylhexanol: 0.17%    -   MMA: 0.1%

Example 2 Preparation of 2-ethylhexyl methacrylate by a batch process

A 20 m³ stirred tank reactor with a stirrer, distillation column andcondenser is initially charged with 8030 kg of methyl methacrylate(MMA), 7890 kg of 2-ethylhexanol, 364 g of hydroquinone monomethyl etherand 36 g of 4-hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl asinhibitors, and also 90 kg of 2-ethylhexyl titanate as a catalyst, andstirred with constant introduction of a gas mixture composed of 95% N₂and 5% O₂ (10 m³/h).

To stabilize the column, over the entire reaction phase, a total of 908kg of MMA which contain 908 g of hydroquinone monomethyl ether and 90 gof 4-hydroxy-2,2,6,6-tetamethylpiperidine 1-oxyl are metered into thecolumn reflux. The mixture is heated to boiling temperature (approx.100° C.). At a constant reflux ratio of 1:1, the methanol-MMA mixtureformed is drawn off.

During the reaction phase, 2800 kg of MMA are metered continuously intothe reactor. After approx. 3 h, the reaction is very substantiallycomplete.

Up to a product temperature of 130° C., excess MMA is subsequently drawnoff at a reflux ratio of 0.2:1; the pressure is lowered continuouslydown to 40 mbar. The gas mixture introduction in the vacuum phase isreduced to 5 m³/h. The vacuum is broken when a pressure of 40 mbar isattained at a bottom temperature of 110° C. or higher (approx. 3 h). Thetank contents are subsequently cooled to 80° C. and pumped into anintermediate tank.

The crude product is pumped out of the intermediate tank into athin-film evaporator. The pure product is removed there at a pressure of5 mbar. The product is condensed and collected in a tank.

A pure product is obtained with the following composition (determined bygas chromatography):

2-EHMA 99.1%

2-ethylhexanol 0.3%

MMA 0.5% Example 3 Preparation of 2-ethylhexyl methacrylate by acontinuous process

2-Ethylhexyl methacrylate is prepared continuously in a stirred tankbattery consisting of three stirred tanks connected in series, each ofcapacity 2.1 m³, comprising a first column unit for removing themethanol-methyl methacrylate mixture formed and a second column unit forremoving low-boiling components. The stirred tank battery is suppliedcontinuously with 700 l/h of 2-ethylhexanol, 600 l/h of methylmethacrylate (MMA) and 15 kg/h of a 50% solution of 2-ethylhexyltitanate in MMA, which has been stabilized with 525 ppm of hydroquinonemonomethyl ether.

Additionally metered into the system are 15 l/h of 3.5% hydroquinonemonomethyl ether in MMA via reaction stage 1.

The individual reaction stages are supplied with stabilization air of ineach case 450 l/h of fresh air. The vapours from the stirred tank whichhave been freed of methanol in the first distillation column are fed tothe 1st stirred tank via the column bottom.

Under these reaction conditions (pressure 500 mbar), a reactiontemperature of 107° C. is established in the first stirred tank. Thereaction temperature is 125° C. in the 2nd stirred tank and 136° C. inthe 3rd stirred tank.

The methanol formed is drawn off continuously as a methanol-MMA mixtureat a rate of 240 l/h via the first distillation column with acirculation evaporator. The effluent of the 1st reaction vessel ispassed on into the 2nd reaction vessel, and the effluent of the 2ndreaction vessel into the 3rd reaction vessel.

The effluent of the 3rd reaction vessel is fed continuously to thethin-film evaporator of the low boiler column, in which unconverted2-ethylhexanol, MMA and methanol are drawn off as distillate (350 l/h)and fed back to the first distillation column.

The bottom effluent of the low boiler column is 1000 kg/h and has acomposition of 98.1% 2-ethylhexyl methacrylate, 1.0% MMA and 0.7%2-ethylhexanol and, to a smaller degree, high boilers and reactants.

Emulsion polymerization of 2-ethylhexyl methacrylate

The 2-ethylhexyl methacrylate prepared according to Examples 1 to 3 wasin each case polymerized by emulsion polymerization.

To this end, 400 g of the 2-ethylhexyl methacrylate prepared wereprocessed to an emulsion by means of an Ultra-Turrax at 4000 rpm for 3minutes with

 18 g of sodium lauryl sulphate 0.6 g of K₃PO₄•3 H₂O in 50 g of dist.water 0.6 g of KH₂PO₄ in 50 g of dist. water 0.06 g  of ammoniumperoxodisulphate (APS) in 50 g of dist. water 373 g  of dist. water.

The emulsion was transferred to the initial charge of a polymerizationvessel which was cooled to circulation temperature 5° C. Simultaneously,nitrogen was introduced into the reaction mixture which was stirred at100 rpm. Subsequently, the metered addition of a solution of 0.072 gFeSO₄.7 H₂O in 100 g of dist. water over 20 hours was commenced. Afterthe end of feeding, 24 g of Triton×305 (70% strength) in 24 g of dist.water were added. Subsequently, the dispersion was filtered through astainless steel screening fabric with MW 0.09 mm.

The specific viscosity η_(spec/c) was determined based on DIN 51562 inTHF as a solvent. The concentration was selected so as to achieve arelative viscosity in the range of 1.1-1.2. The particle radius wasdetermined as the r_(N5) value with an N5 Submicron Particle SizeAnalyzer from Beckman Coulter according to the manufacturer'sinstructions.

The analytical data of the monomers and of the polymer dispersionsobtained from the monomers can be taken from the table which follows:

EHMA characterization and resulting homopolymer Air/O2 Total O2 inputcontent [%] [l/kg EHMA] Monomer on including Example preparationdegassing 1 18 2.4 2 5 0.2 3 18 0.2 Stabilization of theCharacterization of the Dry monomer used TempolMA* resulting polymerdispersion rN5 content Example HQME [ppm] Tempol [ppm] [PPM] η_(spec/c)in THF [ml/g] [nm] [%] pH 1 23.5 — — 715 66 39.7 7.1 2 10 <1 4.5 1464 5239.6 7.2 3 1 <1 <1 1369 69 39.6 7.2 *TempolMA is a conversion productinevitably formed in the transesterification by reaction of Tempol withmethyl methacrylate.

1. A process for preparing a (meth)acrylate of formula (I)CH₂═C(R¹)—CO—O—R²   (I) wherein R¹ is hydrogen or methyl, and R² is asaturated or unsaturated, linear or branched, aliphatic or cyclic alkylradical having 6 to 22 carbon atoms, or a (C₆-C₁₄)-aryl-(C₁-C₈)-alkylradical, the process comprising reacting a (meth)acrylate of formula IICH₂═C(R¹)—CO—OR³   (II) wherein R¹ and R³ are each independentlyhydrogen or methyl, with an alcohol of formula (III)HO—R²   (III) wherein R² is a saturated or unsaturated, linear, branchedor cyclic alkyl radical having 6 to 22 carbon atoms, or a(C₆-C₁₄)-aryl-(C₁-C₈)-alkyl radical, in the presence of an amount of asuitable catalyst which catalyses the reaction and in the presence of anamount of a phenolic polymerization inhibitor or a combination of two ormore phenolic polymerization inhibitors which is sufficient to inhibitundesired polymerization, to form a reaction mixture, the reactionundertaken with introduction into the reaction mixture resulting fromthe reacting, of an amount of oxygen or of an oxygenous gas mixturesufficient to inhibit undesired polymerization, wherein a specific totaloxygen input is less than or equal to 1.0 l/kg, measured in liters ofoxygen per kilogram of (meth)acrylate of formula (I), where the volumeof oxygen introduced is calculated at a temperature of 25° C. and apressure of 101,325 pascal, and wherein the reacting is performedcontinuously.
 2. The process according to claim 1, wherein the reactingof the alcohol of formula (III) to (meth)acrylate of formula (I) iscarried out in a reaction vessel having a reactor volume of greater thanor equal to 0.25 m³.
 3. The process according to claim 1, wherein the(meth)acrylate of formula (I), after the reacting has ended, is isolatedby distillation.
 4. The process according to claim 1, the wherein R² isa linear or branched alkyl radical having 8 to 12 carbon atoms or a(C₆-C₁₂)-aryl-(C₁-C₄)-alkyl radical.
 5. The process according to claim1, the wherein R² is a 2-ethylhexyl radical.
 6. The process according toclaim 1, wherein the catalyst is tetraisopropyl titanate,tetrakis(ethylhexyl) titanate, zirconium acetylacetonate, a dialkyltincompound, a lithium compound, optionally in combination with a calciumcompound, or an acid.
 7. The process according to claim 1, wherein thephenolic inhibitor is hydroquinone and/or hydroquinone monomethyl ether.8. The process according to claim 1, wherein the oxygenous gas mixtureintroduced into the reaction mixture is oxygenous lean air having acontent of less than or equal to 5% oxygen (v/v).
 9. The processaccording to claim 1, wherein the specific total oxygen input is lessthan or equal to 0.5 liter of oxygen per kilogram of product of theformula (I).
 10. The process according to claim 1, wherein the specifictotal oxygen input is less than or equal to 0.3 liter of oxygen perkilogram of product of the formula (I).
 11. The process according toclaim 1, wherein the specific total oxygen input is less than or equalto 0.2 liter of oxygen per kilogram of product of the formula (I). 12.(canceled)
 13. The process according to claim 1, wherein the catalyst istetraisopropyl titanate or tetrakis(ethylhexyl) titanate.